As Texas solicitor general, Cruz cited Harvard professor Laurence Tribe as “a prominent commentator” in his brief for Medellin v Texas, a case the senator invariably mentions on the stump.

In Thursday night’s Republican debate, under fire from Trump, Cruz changed his tune about Tribe, who taught him constitutional law at Harvard, calling him “a left-wing judicial activist, [a] Harvard law professor who was Al Gore’s lawyer in Bush versus Gore … a major Hillary Clinton supporter”.

In that case, Cruz successfully argued that Texas should be allowed to execute a Mexican national convicted of rape and murder who had not been advised his rights to contact his country’s consulate under international law.

Cruz also cited Tribe in his brief for District of Columbia v Heller, a landmark 2008 case in which the court held that the second amendment provided for an individual right to bear arms.

Cruz wrote: “For example, although arguing for a narrow construction of the amendment, Professor Laurence Tribe has squarely concluded that the second amendment provides a “right (admittedly of uncertain scope) on the part of individuals to possess and use firearms in the defense of themselves and their homes.”

As the 2016 presidential campaign enters a crucial stage, Trump has attacked on the citizenship question. Last weekend, Tribe suggested in an interview with the Guardian that the issue was “unsettled”.

On Saturday Tribe told the Guardian, via email, that he found Cruz’s citations in his supreme court briefs “interesting”.

“Apparently Senator Cruz doesn’t think I’m too far left for him to cite me in his supreme court brief on behalf of 31 states as a principal authority on constitutional interpretation – when it suits his purposes,” said Tribe.

“Somehow I don’t feel particularly complimented.”

Trump has become a fan of Tribe, praising him an interview on NBC’s Meet The Press as “a constitutional expert, one of the true experts” and citing him in Thursday’s debate .

Tribe’s views have become the bedrock of Trump’s argument that Cruz’s Canadian birth will leave Republicans on uncertain legal ground if the Texas senator is the presidential nominee.

“There’s a big question mark on your head. And you can’t do that to the party. You really can’t,” Trump said during the debate, in Charleston.

The constitution requires the president to be “a natural-born citizen”. Speaking to the Guardian last week, Tribe noted that it was unclear whether someone born to an American mother abroad, as Cruz was in Canada, met that qualification.

Although Tribe said “the kind of judge that I admire and Cruz abhors – a ‘living constitutionalist’ who believes that the constitution’s meaning evolves with the needs of the time” would find Cruz eligible, he thought an “originalist judge” who shared the Texas senator’s legal philosophy might not.

On Saturday a Cruz spokeswoman said, in a statement: “[Tribe’s] agreement on a matter of the constitution gives further credibility to Cruz’s arguments – even liberals recognize the straightforward facts about the constitution and the law.”

Regarding the Texas senator’s recent statements about his former law professor, the spokeswoman added: “Cruz is stating the facts: Tribe is a liberal and he supports Hillary Clinton so his attacks are clearly politically motivated. That is a surprise to no one.

“Regardless, their agreement on certain matters of the constitution enhances Cruz’s credibility on the issues at hand.”